The present invention relates generally to the formation of thin film structures by ion beam sputter deposition and, more particularly, to the fabrication of thin film structures and magnetoresistive sensors wherein the ratio of the atomic mass of the ion beam sputter gas to the atomic mass of the target material is controlled to produce thin film structures having desired properties.
It is well-known in the prior art to utilize RF or DC magnetron sputter-deposition apparatus for fabrication of thin film devices such as magnetic recording sensors and storage media. Such sputter devices are characterized by crossed electric and magnetic fields in an evacuated chamber into which an inert, ionizable gas, such as argon, is introduced. The gas is ionized by electrons accelerated by the electric field, which forms a plasma in proximity to a target structure. The crossed electric and magnetic fields confine the electrons in a zone between the target and substrate structures. The gas ions strike the target structure, causing ejection of atoms that are incident on a workpiece, typically a substrate on which it is desired to deposit one or more layers of selected target materials.
In prior art conventional sputtering devices relatively high operating pressures are utilized in order to obtain films having low internal stress which results in non-directional sputtering flux at the substrate. However, this non-directional flux introduces manufacturing process difficulties as device dimensions become increasingly smaller. For example, in a metal liftoff process with bilayer photoresist, a 0.5 micrometer (um) undercut is required to prevent fencing. As line dimensions decrease below 5.0 um, an undercut of this magnitude not only introduces adhesion problems with the bilayer photoresist, but it becomes increasingly difficult to accurately define track width in high density storage media. Additionally, high-pressure sputtering can result in porous films having columnar zone 1 type microstructure. A porous microstructure degrades physical properties of the films, such as resistivity, and introduces long-term stability problems in the films.
It is known in the prior art to utilize secondary beam deposition, also referred to as ion beam sputter deposition, in certain applications to overcome some of the difficulties encountered with conventional RF/DC sputter techniques. Several aspects of ion beam sputter deposition differ from conventional sputter processes and provide significant advantages. The use of low background pressure results in less gas incorporation in the deposited films and less scattering of sputtered particles, i.e., longer mean-free path, during the transit from the target to the substrates. Control of the ion beam directionality provides both a variable angle of incidence of the beam at the target and a controllable angle of deposition at the substrates. A nearly monoenergetic beam having a narrow energy distribution provides control of the sputter yield and deposition process as a function of ion energy and enables accurate beam focusing and scanning. The ion beam is independent of target and substrate processes which allows changes in target and substrate materials and geometry while maintaining constant beam characteristics and allowing independent control of the beam energy and current density. Finally, since the target and substrate are not part of the RF/DC circuit, substrate heating is minimized reducing cooling requirements. However, a major hurdle encountered with ion beam sputter deposition is the resulting high internal stress in the deposited films. This is due primarily to high energy bombardment during deposition of the film by backscattered neutral atoms as a result of high ion beam energies and the relatively low operating pressure utilized in the ion beam deposition apparatus.
Apparatus and methods fop producing a thin film deposit on a substrate utilizing ion beam sputtering are described, for example, in U.S. Pat. No. 4,923,585, to Krauss et al. Krauss et al discloses the use of a computer controlled, single ion beam with a quartz crystal monitor to produce deposited films of arbitrary composition as well as layered structures of arbitrary thickness from multiple targets of elemental components of the desired films and layered structures. While Krauss et al discloses the use of a high energy ion beam to obtain films of desired composition, the problems associated with films having high internal stress or the effects of backscattered neutral beam atoms on film purity and characteristic properties are not addressed.